Keyswitches for keyboards may generally be divided into two basic types, conductive and capacitive. While both types have many common features the design problems associated with the two types differ. The two major design problems in conductive keyswitches are the prevention of oxides forming on the contact surfaces and the elimination of contact bounce.
In capacitive keyboards the formation of an oxide film on the switch members is unimportant since it is desirable to have the members insulated from each other. While contact bounce is not a major problem in capacitive keyboards another design problem arises in that tilting of the switching members at different angles with respect to the printed circuit board results in different capacitance for the same keyswitch struck at different angles. Thus, solutions in designing conductive keyboards may be difficult to utilize in the design of capacitive keyboards. Common to both types of keyboards are error signals in flat keyboards without keycaps caused by finger pressure on the edges of two adjacent keys where both keys produce signals although finger pressure was applied in the space between keys on a crowded keyboard.
The keyboard according to the invention was designed to minimize the occurance of tilted actuators in a keyboard of limited thickness and reduced cost.
A first type of prior art capacitive keyboard shown in U.S. Pat. Nos. 3,710,209; 3,751,612; 3,643,041 and 3,900,712 is the "snap action" type where a conductive dome is aligned above the conductor pairs of a printed circuit board and where downward pressure on the top of the dome causes the dome to arc downward toward the printed circuit board. In all the examples of this type of switch with the exception of the type shown in U.S. Pat. No. 3,900,712, the rounded shape of the moving switch member, or actuator, establishes weak capacitance with the printed board conductors. The latter prior art device uses a plastic dome provided with a foam pad actuator. The construction of the plastic dome apparently makes it necessary to use a flat keyswitch to depress it due to the obvious tendency of the plastic dome to depress irregularly, thereby bringing the moving contact toward the printed circuit board at an angle and off center.
A second type of keyswitch is exemplified by U.S. Pat. No. 3,797,630. In this prior art device a conductive elastomer pad is moved toward the printed circuit board with an actuator and is maintained above the printed circuit board by the use of a spring. Such a configuration requires a keyboard of a considerable height and a number of complicated parts to be manufactured. In addition, such keyboard needs some provision for keeping liquids from penetrating the space between keycaps and flowing onto the printed circuit board if a reasonable keyboard lifetime is desired.
A third type of prior art keyboard as shown in U.S. Pat. Nos. 3,968,488 and 3,696,908 uses a thin metal sheet, appropriately cut to form a plurality of springs surrounding a disc used as the actuator or movable capacitor plate of a capacitance switch. In order to maintain the metal sheet spaced from the printed circuit board a separating lattice must be used. In addition, the spring thus formed does not have a uniform spring constant around the periphery of the disc-shaped activator. Thus, there is a tendency for an eccentrically depressed actuator to pivot prior to moving toward the printed circuit board, thereby resulting in non-uniform capacitance for a given key struck at various degrees of eccentricity. It is therefore necessary to provide additional equipment to insure that the actuators formed from the metal sheet are moved by devices that press the discs concentrically with a surface that is maintained parallel to the printed circuit board surface.
An interesting prior art conductive keyboard is shown in U.S. Pat. No. 4,017,848. In this device a plurality of parallel conductive strips is formed on a printed circuit board. A plurality of additional such strips are affixed to the side of a plastic sheet that faces the printed circuit board. The printed circuit board and sheet are separated by an insulating liquid sealed between them. Finger pressure on the sheet toward the printed circuit board causes the liquid to be pushed aside and permits one of the conductors on the plastic sheet to contact the printed circuit board conductor beneath it. It would be rather difficult to adapt such a keyboard to capacitive operation because different finger pressures would produce different contact areas, resulting in non-uniform capacitance.
A further prior-art keyboard type is shown in U.S. Pat. No. 3,668,698. In this device a flat conductive silicone rubber sheet has a right cylindrical projection above each spaced conductor pair on the printed circuit board. The sheet is held spaced from the printed circuit board by an insulating matrix. An electret film between the matrix and the printed circuit board insulates the printed circuit board conductors from the conductive rubber projections. As with the keyboards using a properly cut metal sheet to form the actuators the keyboards of U.S. Pat. No. 3,668,698 requires a separate matrix spaced from the printed circuit board to hold the actuators. In addition a small angular deflection at the top of the cylindrical projection causes the bottom end to swing through a large arc, so that misalignment with the spaced conductors of the printed circuit board is likely.